1. Field of the Invention
The present invention relates to a colorimetric apparatus and colorimetric method capable of colorimetry.
2. Description of the Related Art
Recently, due to their high image quality, inkjet printing apparatuses are often used as industrial printing apparatuses for dealing with printed materials. The inkjet printing apparatuses require high reproducibility for even a small color tone difference or slight density unevenness of an output image. The inkjet printing apparatus sometimes includes a plurality of printheads and nozzle arrays for discharging ink of the same color. In this case, the discharge characteristic may differ between the printheads or the nozzle arrays owing to variations of the heat generation amount of a heater for discharging ink or variations of the diameter of a nozzle for discharging ink, failing to obtain a desired color tone in an output image. To cope with such color tone difference, calibration is generally executed. The calibration changes a gamma table used in gamma correction processing for correcting the discharge characteristic of a printhead. For example, a patch image is printed on a sheet using a plurality of printheads and nozzle arrays, and the gamma table is appropriately set again based on the printing result.
The inkjet printing apparatus is often used for proof in printing because the running cost in printing is lower than in another type of printing apparatus. The poof aims at outputting the same result as the tint of a final printed material, so color proof is performed to adjust the color to that of the final printed material, in addition to the above-mentioned calibration. For example, a profile is created so that the same color as a target CMYK or RGB input is output by taking account of the apparatus-specific color reproduction characteristic and the characteristics of a sheet used in color proof. Printing based on the created profile can output the same tint as that of a final printed material.
A colorimeter is used in the above-described calibration and profile creation. It is common to use a reflection spectral colorimeter capable of measuring reflected light in the visible light region for each wavelength band and outputting a chromaticity complying with a colorimetric system such as CIEL*a*b* in accordance with the measurement result. In recent years, inkjet printing apparatuses which include a colorimetric sensor and can perform seamless colorimetry for a printed material are also used.
Recently, there are various types of sheets (printing media) designed for inkjet printing apparatuses, including art paper and Japanese paper. A sheet containing a fluorescent brightener for making output look whiter is sometimes used. When the user selects a sheet from various types of sheets available from third parties and is to print, he needs to create a profile in accordance with an inkjet printing apparatus to be used because the sheet vender does not always provide a profile optimum for all inkjet printing apparatuses.
A fluorescent brightener-containing sheet differs greatly in appearance depending on the intensity of ultraviolet light contained in a light source, even under a light source which looks the same. This is because light in the ultraviolet (UV) wavelength band contained in the light source is excited by the fluorescent brightener contained in the sheet and reflected as visible light. For a light source containing no ultraviolet light region and a sheet containing no fluorescent brightener, an intensity in the wavelength band of the light source becomes equal to one in the wavelength band of light reflected by the sheet. However, when a fluorescent brightener-containing sheet is measured under a ultraviolet light region-containing light source, light in the wavelength band of ultraviolet light is excited by the fluorescent brightener and reflected as visible light. For this reason, an intensity in the wavelength band of ultraviolet light affects the wavelength band of visible light. Even if intensity in the visible light region of the light source remains unchanged, the difference of the spectral intensity in the ultraviolet light region makes the tint look different. This is known as a metamerism phenomenon, in which the tint of an image printed on a sheet looks different depending on the type of observation light source used to evaluate the image. Under these circumstances, in proof and production printing requiring strict color management, light source conditions such as the presence/absence of the ultraviolet (UV) blocking filter of a measurement light source are defined by international standards (for example, ISO13655: 2009, and ISO3664). Recently, the spectral intensity of an ultraviolet light source is also defined to make a measurement light source and environment light source match each other more strictly.
To evaluate an optimal color under an environment light source, a plurality of profiles are created based on the results of measurement under a plurality of measurement light sources. Measurement of the spectral reflectance considers various light source conditions, so the colorimeter is often configured to allow the user to arbitrarily attach or detach an UV blocking filter. Some colorimeters can measure a color under desired conditions by using an ultraviolet light source and visible light source as a plurality of physical light sources and switching between these light sources. Such a colorimeter can be used to easily obtain colorimetric values under a plurality of light sources in profile creation.
In general, the number of patches to undergo colorimetry in calibration is about 300, and the number of patches to undergo colorimetry in profile creation is as many as 1,000. Further, every time one measurement condition is added, the amount of colorimetric result data is doubled. That is, every time the measurement light source is switched, a buffer area for saving a measured spectral reflectance needs to be ensured for the number of patches mentioned above. Generally, the frequency at which colorimetry under a plurality of light source conditions is performed is much lower than the frequency at which colorimetry under a fixed light source condition (for example, closed-loop calibration) is performed. Owing to low-frequency measurement, a large buffer area needs to be ensured. Japanese Patent Laid-Open No. 2009-220290 discloses an arrangement in which a colorimeter is directly controlled by a printer via a USB connection. However, as the number of patches increases, colorimetric result data consumes the limited storage area of the USB.